Phthalic anhydride is an important intermediate chemical in the chemical industry. One important use is in the production of phthalates such as di-isononyl or di-isodecyl phthalates, which are used as plasticisers, typically for polyvinyl chloride. Phthalic anhydride has been produced on an industrial scale for many years and has generally been produced by the vapour phase oxidation of ortho-xylene with an oxygen-containing gas, such as air, by passing a mixture of ortho-xylene and the oxygen-containing gas over an oxidation catalyst.
A typical plant for the production of phthalic anhydride comprises a raw material delivery section, a raw material mixing section in which a hot mixture of the oxygen-containing gas and ortho-xylene vapour is prepared and a mixture delivery section for feeding to a reaction system comprising the reactor which typically consists of reactor tubes containing catalyst. The components of these sections are known as the process equipment. The reaction is exothermic and the temperature of the reactor tubes is controlled by a temperature control fluid, such as molten salt, flowing around the tubes.
After the reaction, the crude phthalic anhydride that has been produced passes to a cooling stage where it is cooled, generally by a gas cooler, passed to optionally a liquid condenser and finally to a switch condenser. Finally, the condensed phthalic anhydride is subjected to a purification or finishing step.
The efficiency of a phthalic anhydride plant is measured in terms of the number of grams of ortho-xylene that can be processed for each cubic meter of air that is fed to the raw material section (known as the loading). The greater the amount of ortho-xylene, the greater is the efficiency of the facility. Considerable attempts have been made over the years to increase the loading, and loadings above 80 gram/Nm3 of ortho-xylene in air have been reported.
One difficulty in the manufacture of phthalic anhydride is that, at the temperatures required for the reaction of air and ortho-xylene the mixture becomes flammable and explosive at a loading above 44 gram of ortho-xylene per normal cubic meter of air. Accordingly, great care must be taken to avoid or reduce the likelihood of explosions. When an explosion occurs and the flame velocity exceeds the velocity of sound, this supersonic explosion is called a detonation. Otherwise, at subsonic flame velocities, it is called a deflagration. By the provision of an adequate number of escape ducts, such as chimneys, sealed off by rupture discs, at critical locations, the occurrence of a detonation is avoided, while the burning gas from a deflagration is relieved to a safe location. One or more rupture discs are conveniently located on the ortho-xylene vaporizer, at the reactor inlet and outlet, and on downstream equipment and the sections of the piping operating within the flammability limits. These rupture discs can be of any suitable design, although reverse buckling or bending rod type are preferred. One of the areas in a phthalic anhydride facility that is prone to a deflagration is the raw material mixing section, where the ortho-xylene and the air are mixed. One of the reasons for a deflagration to occur is if there is incomplete vaporisation or condensation in the vapour/air mixture at the time when it reaches the oxidation catalyst. Other reasons can be poor mixing of the heated ortho-xylene and the heated air, inhomogeneity in the composition of the mixture, discharges from the build-up of static electricity, or the decomposition of peroxides formed from feed impurities like cumene or styrene. The present invention is concerned with reducing or minimising the likelihood of a deflagration of an explosion occurring.
In a typical commercial process the generation of a feed gas mixture has to date been performed as follows. Process air is sucked in from the surroundings through a filter by means of a blower, and compressed to a pressure level which allows the conveyance of the air stream through the phthalic anhydride plant. This process air stream is heated in a heat exchanger disposed downstream of the blower. Parallel thereto, liquid ortho-xylene from a storage tank is brought to a preliminary pressure by means of a pump and passed through a basket type filter and a preheater before it is fed to an evaporator, vaporizer drum or spray drum. In the evaporator, the preheated ortho-xylene is injected in liquid form into the heated air stream parallel to the air flow, by means of a nozzle system. The fine ortho-xylene droplets completely evaporate in the air stream, and a further smoothening of the radial concentration and temperature profiles in the gas stream is achieved by means of a homogenisation stage (a homogenizer section comprising e.g. a static mixer). This feed gas subsequently enters the reactor, typically a tubular reactor comprising of tubes filled with catalyst to provide a catalyst bed, where a partial oxidation of ortho-xylene with the oxygen takes place to form phthalic anhydride.
This process for the generation of feed gas has successfully been used, but with the successive introduction of higher ortho-xylene loads in the air stream (above 80 g ortho-xylene per Nm3 air) the process has shown potential weaknesses with regard to the explosion safety of the raw material section of the plant. The lower explosion limit of a gaseous mixture of ortho-xylene and air is about 44 g of ortho-xylene per Nm3 of air. It has been found that the minimum energy required for igniting the mixture is greatly decreased with increasing ortho-xylene load, and therefore the desire to increase the ortho-xylene loading increases the possibility of an explosion. However, to a great extent, the economics of the overall phthalic anhydride production process depends upon increasing the load of ortho-xylene per Nm3 air. It is therefore of basic importance that plants with a loading in the range of 80 g ortho-xylene/Nm3 air to 120 g ortho-xylene/Nm3 air must be operated safely.
U.S. Pat. No. 6,984,289 B2 relates to a process for the production of phthalic anhydride by the oxidation of ortho-xylene with air and with a loading of 80 g to 100 g of ortho-xylene per Nm3 of air. This higher loading is said to be made possible by complete evaporation followed by superheating of the ortho-xylene prior to admixture with air. U.S. Pat. No. 4,435,581 discloses a process wherein naphthalene is first completely evaporated before bringing the vapours in contact with the air stream in a reactor containing a fluidised bed of oxidation catalyst. DE 20 2005 012 725 U1 provides a system in which ortho-xylene is sprayed through nozzles into an air stream in which the flow cross-section of the air feed tube is reduced downstream of the spray nozzles, so that vapour velocity and turbulence are increased, thereby improving the mixing of the reaction components, and in this way the risk of explosion is reduced. DE 20 2005 012 725 U1 also provides a cone-shaped perforated screen at either side of the spray nozzles to divert the pressure wave from an explosion occurring in the evaporation section towards the rupture discs, thereby protecting the equipment upstream and downstream from these screens from damage by a shock wave. These screens assist also in homogenising the flow of air and the flow of the air/ortho-xylene mixture.
U.S. Pat. No. 4,119,645 also relates to a process for the production of phthalic anhydride by the oxidation of a mixture of ortho-xylene with air, but is silent about how the mixture is produced and passed to the oxidation reactor. U.S. Pat. No. 4,119,645 is not concerned with the homogeneity of the mixture or how to preserve it until it reaches the catalyst. Patents GB 1550036 and GB 1239803 also relate to processes for the production of phthalic anhydride by the oxidation of a mixture of ortho-xylene with air. The processes operate at loadings of ortho-xylene in air that are below or barely above the lower explosion limit and much lower than current industrial practice. These processes are therefore much less sensitive to an inhomogeneity in the ortho-xylene/air mixture. GB 1550036 and GB 1239803 are silent about the production of the ortho-xylene/air mixture and the passing thereof to the oxidation reactor. These documents are not concerned with the homogeneity of the mixture or how to preserve it until it reaches the catalyst.
United States patent application US 2003/0013931 A1 relates to a process and apparatus for producing a homogeneous mixture of ortho-xylene vapour in air, as feed to an oxidation reactor for the production of phthalic anhydride. US 2003/0013931 A1 is concerned with rapid vaporisation of the ortho-xylene into the air stream, and employs special spray nozzles to that effect. The spraying is performed in a chamber bounded by side walls heated to a temperature above the boiling point of ortho-xylene, such that droplets of ortho-xylene which impinge on the tube wall are vaporised immediately and do not deposit as a liquid film. US 2003/0013931 A1 is not concerned with avoiding condensation on surfaces in contact with the ortho-xylene/air mixture as it passes to the catalyst in the reactor. It is silent about the surfaces between the end of the heatable double-walled tube and the top tubesheet of the reactor. US 2003/0013931 A1 is also silent about any rupture disks that may for safety reasons be provided on the inlet head of the oxidation reactor, in the raw material mixing section or in the section delivering the mixture to the catalytic reactor. These rupture disks are safety devices and in a heatable double-wall version would not be readily able to perform their critical function. US 2003/0013931 A1 is not aware of the problems of possible condensation of ortho-xylene on the internal surfaces of these rupture disks or the flanges and piping connecting thereto. It is therefore not concerned with the temperature of internal surfaces of rupture disks or other equipment elements up to the inlet of the catalyst bed in the oxidation reactor.
It is important that a homogenous mixture of ortho-xylene and air is formed for feeding to the reactor and this may be accomplished by enhancing the rate of ortho-xylene vaporisation. As is described in our co-filed UK application reference GB 0718994.7, we have found that this may be accomplished by employing a particular nozzle system, and a particular set of conditions within the nozzle, to spray the ortho-xylene into the hot air, and in particular GB 0718994.7 is concerned with a system for mixing ortho-xylene with an oxygen-containing gas, which system comprises an ortho-xylene evaporator or vaporiser vessel fed with a stream of oxygen-containing gas and provided with at least one lance projecting into the stream of oxygen-containing gas, which lance is provided with at least one metal spray nozzle adapted to inject droplets of liquid ortho-xylene into the stream of oxygen-containing gas concurrently with the direction of flow of the stream of oxygen-containing gas, in which the metal at the surface of the spray nozzle, that in use is in contact with the liquid ortho-xylene, has a hardness expressed as a Vickers hardness number according to ASTM E92-82 of at least 200, preferably at least 250 and more preferably at least 600. The spray nozzle is preferably made of hardened steel, more particularly surface hardened austenitic stainless steel. The desired surface hardness is preferably obtained by nitriding the nozzle surface, more preferably by cold nitriding such as by Kolsterising®.
In addition, the applicants co-filed application reference GB 0718994.7 describes a particular sealing system to prevent the leakage of liquid ortho-xylene at undesired locations from the spray nozzle system. This spray nozzle system, including the sealing system preferably comprising an annealed copper seal ring, is particular useful when used in combination with a specially designed oxygen-containing gas feed system and a particular design of oxygen-containing gas and ortho-xylene mixing system.
In operation, ortho-xylene is preheated to about 140° C. under elevated pressure, flow metered with mass flow meters, and forced into a spray nozzle configuration for injection into the heated oxygen-containing gas, which is typically air. The hot liquid ortho-xylene is thus sprayed as a fine mist into the hot air upon which the ortho-xylene is vaporised. The present invention is concerned with maintaining the ortho-xylene in the vapour phase. Furthermore, it is important that the ortho-xylene does not coalesce or condense and form liquid deposits within the raw material section of the plant, to reduce the risk of explosion when liquid deposits are formed. When ortho-xylene is allowed to condense on internal surfaces in the equipment up to the inlet tubesheet of the tubular reactor, relatively large droplets may come loose from the surface and be entrained by the mixture of ortho-xylene vapour and the oxygen-containing gas. Entrained larger droplets may not be totally vaporised by the time they reach the oxidation catalyst bed, and cause a local excessive reaction, increasing the risk for a runaway reaction and possible catalyst damage, and for an explosion. When such entrained larger droplets become totally vaporised but only just before they reach the catalyst bed, they may still be causing smaller volumes in the vapour/gas mixture in which the concentration of ortho-xylene is higher than average. Such an inhomogeneity may also cause a local excessive reaction when reaching the catalyst, and trigger an explosion. In order to reduce the explosion risk it is therefore not only important to rapidly vaporise the ortho-xylene that is sprayed into the oxygen-containing gas, it is also important to avoid condensation of ortho-xylene from the mixture on internal surfaces of the equipment from the production of the vapour/gas mixture up to the inlet tubesheet of the reactor.